\partcombine problems

[Jefferson dos Santos Felix]

Recently I have use the \partcombine to create SATB scores, but I have 
some problems:

1: I want to put automatic stem on both direction, when soprano and alto 
or tenor and bass plays the same notes. Actually, I use <<c\\c>> , e.g., 
every time I need both stems. The idea is to create a Staff property to 
control both stems on  unissono parts (like this: \set 
Staff.PartCombineBothStems =  ##t), to activate both stems on unissono 
parts, with ##f by default.

2: Lilypond doesn't combine notes with inteval more than 1 octave.... 
Why? Is it a bug, or an engraver rule?

3: Where is the rests combined with notes? I think you keep out it on 
2.3.22. It's cause a serious interpretation error on this example. The 
last note on treble clef, e.g.: is it play by soprano or alto???

4: How can I set default stem direction on \partcombine? I set up treble 
with \stemUp, and bass with \stemDown, but there are stems in 
incorrectly directions.


%%BEGIN
global = {
   \key c \major
   \time 4/4
   \set Staff.autoBeaming = ##f
   \set Staff.printPartCombineTexts = ##f
   \set Staff.minimumVerticalExtent = #'(-8 . 8)
}

soprano = \relative c'' {
   <<c4\\c>> c b a g f' e d c c2. r4
}

alto = \relative c' {
   s4 e f f e c' g f e e f e g
}

tenor = \relative c' {
   <<c4\\c>> c b a g g a b c c2. r4
}

bass = \relative c {
   s4 c c c c c a g c c g c g
}

\score
{
   <<
       \new Staff
       {
           \partial 4
           \global
           \stemUp
           \partcombine \soprano \alto
       }
      
       \new Staff
       {
           \clef bass
           \global
           \stemDown
           \partcombine \tenor \bass
       }
   >>
}
%%EOF

That's all, people...

Jeff